In general, power over Ethernet (PoE)-refers to a technique that is outlined in the IEEE Std 802.3™-2005 clause 33 (the PoE standard) for delivering power via Ethernet cabling to a powered device, which is an electronic device adapted to derive power from the Ethernet cabling. PoE eliminates the need for a separate power source for powered devices. Powered devices may include voice over Internet protocol (VoIP) telephones, wireless routers, security devices, field devices to monitor process control parameters, data processors, and the like.
In general, power sourcing equipment delivers power to one or more powered devices via a twisted pair network cable, such as an Ethernet cable. The PoE standard specifies that the power sourcing equipment should provide a power supply voltage having a range of 36 to 57 volts direct current (DC) and having a current that is limited to less than 400 mA. The use of extended cable lengths and transformers in some applications may induce transient voltage and current surges far in excess of 57 volts and 400 mA.
Conventionally, to manage such transients and other power events, many powered devices include a high voltage transient suppressor, which may protect circuitry of the powered device from high voltages by clamping the input voltage at or below a breakdown voltage threshold during a power surge event. A typical high voltage transient suppressor may be a high voltage zener diode, such as the SMAJ58A surface mount diode, which is commercially available from a wide variety of circuit component manufacturers. The high voltage zener diode is typically connected in parallel with a diode bridge between the positive and negative voltage supply terminals to clamp transient voltages in excess of a threshold to protect associated circuitry. For example, a typical zener diode is a 58-volt zener diode, which is designed to breakdown and conduct current at voltages in excess of 58 volts.
The PoE standard specifies that powered devices are required to survive transient events up to a 1000 volt power surge and to a transient current of 5 A. A transient surge with a duration of 300 nanoseconds to 50 microseconds dissipates considerably more energy than typical zener diodes are rated to withstand. If the high voltage transient suppressor is a diode stack including a plurality of zener diodes arranged in series, the duration of the transient power surge may cause the zener diodes to undergo self-heating. Additionally, typical zener diodes exhibit significant voltage overshoot during high-current events. This overshoot may damage other circuit elements of the powered device.
Therefore, there is a need for improved transient protection in powered devices.